Radioreceiver



Nov. 26,1935.v 'I H. A. WHEELER 2,022,068

' RADIoREcEIvER l File-d Dec. 1,4,4 1951 A @Pr INVENToR m4" @MMMMM F ATToRNEYs4 Patented. Nov. 26, 1935 PATENT" yOFI-ICE zlozaoss nAnrom-:CEIVER Harold A. Wheeler, Great Neck,N. Y., assignor to Hazeltine Corporation Application December 14, 1931, Serial No. 58,803

. 13 Claims.

Thepresent invention relates to an oscillator modulator, and more particularly to 'oscillator l Vmodulators for use in connection with superhet-- erodyne radio receivers. i In a superheterodyne vreceiver it is customary to convert the desired carrier or signal frequency currents into currents of a single predetermined intermediate frequency. The intermediate fre.

quency is usually lower than the carrier frequency 1o to facilitate-the further amplification and selection of the desired signal. This frequencychanging process has, for the best` results, formerly required an oscillator and a modulator or first y detector, each requiring individual tubes."

Although it has beenv proposed to utilize a single tube for the purpose of generating the local or heterodyne frequencycurrents and combining `the current thus produced with the received signal currents, it has been found that' a single tube could not perform both functions eficiently bec'ause of overloading effects of the oscillations,

' especially inthe grid circuit, partly due to the widely-differing oscillation voltages obtained at the various portions of the tuning range ofl the :23 oscillator. Whena signal is applied from a tuned 7 circuit to the grid-of 'an oscillating vacuum tube,

vany appreciable grid current causes Ja damping of thel tuned circuit and a distortion of the sig' nal currents, producing also cross-modulation of the desired signal by strong interfering signals.

, It is the principal object of the present linveri Y tion to produce an oscillator modulator arrange ment in which [the oscillator shall be self-regel ulating to permit the grid circuit-of the modulator to be free from overloading grid current-ff at any point in the tuning range without a critical adjustment of the feed-back and withouty relying upon overloading or saturation effects .in the plate circuit to control the amplitudefof l oscillation.

Itis a further object of the present invention to provide an oscillator modulator circuit in which the self-regulation of the oscillator may i 'be employed without a sacrifice of cnversion'- l'15 gain ln the modulator.v

ing drawing.

For accomplishing the-objectsof the present Y invention, 'the oscillator modulator-of a-super 'heterodyne, for instance,'is providedlwith a plurality of features, each aiding to'pevent the grid of the oscillator modulator tubegoing appreciably positive relativeto the cathode thereof and thus "drawing space current, which would affect the selectivity of the input circuit or distort the received signals.- The rst of lthese features is the provision of a uniform voltage oscillator arrangement in which. 5 the oscillation voltage applied to the control circuit of the modulator shall be substantially constant regardless of the frequency to which the oscillator is tuned. The oscillator may be of a uniform voltage type 'including a plurality 10 of plate feed-back couplings, one of which 'has an effect which'increases with frequency, wheref as another has an effect vwhich decreases with an increase in frequency, as the frequency is varied by the manipulation vof the'tuning means. 15 Such an oscillator is described in copendingapplicatlon of the present inventor for Feed-back circuits, Serial No. 540,581, filed May 28, 1931, of which this application is a continuation in part. 'I'he voltage output, then, of the oscil- 20 lator circuit may bevregulatedv within substantially narrow iimits. which assists lthe regulator action. described hereinafter, 1in preventing thef grid of the tube having impressed f-,thereona -voltage sumcient to cause it to draw "current,-

'I'he second feature is the .provision of a resistor which isjcommon to the plate and grid .cir-

cuits of the oscillator modulator tube for ,the purpose of regulating the grid bias in accordance with the average plate current. This ,resistor is 30 shunted by a radio-frequency by-pass condenser softhat it affects only the average grid bias and is sol arranged that an increase in the average plate current' williresult in an increase in the grid bias, whichy will' tend to reduce the plate 35 current,'re'sulting in apractically stable condi- .tiona The production.- of oscillations tends yto increase the average plate current slightly, re-

sulting in a further increase in the grid bias, as will be described'more fully hereinafter. 40 The third feature is the proportioning ofthefeed-back from the oscillation circuit. This feed'- back is preferable to the cathode leadof the tu'be'so that it is common to both the plate and grid4 circuits.- "Il'hel effective feed-back and the 45 regulating effect of the cathode resistor are so proportioned that with the maximum 'voltage l swing obtained, `due to the eiect of the oscilla'- tionsI upon the ygrid circuit, the grid will not swing positive enough to start drawing grid-cur-.o rent,'and yet the cathode resistance relative to the feed-back" coupling is not suiilcient to stop os- 'cillation. I I- In .order to.' permit the use of a sumciently largeresistor togive a large voltage change 'for 55 prevent'.v the grid being too `negative relative to the cathode, to positively. bias the gridrelative to thel negative bias which would be obtained by the use of the entire resistor. This may be accomplished by' means of a positive bias applied to the' grid through a resistor, the grid being insulated from the grid circuit by a grid condenser. This constitutes the fourth feature of this invention.

' In this application, the control-grid and cathode will be' considered as thc input terminals of a vacuum tube, the plate andlcathode as output terminals, "and the cathode as the common ter-` ulator constructed in accordance with the present invention as utilized in a superheterodyne radio receiver. e

In Fig. l, to which attention is now invited, the

' vacuum tube I I is a common trlode having plate,

grid and cathode. The tuned' circuit comprises coil I2 and condenser I4, in the grid circuit. The feed-back is obtained by coupling coil I2 with coilv I3 in the plate circuit. Condensers I5 and I S are oscillation-,frequency by-pass -condensers 4The resistanceA I1 is the cathode resistance referred -to above, common to grid andplate cir cuits. The batteries 20 and 2I, in series, supply the space current between plate and cathode.

The tap 23 between these ,batteries supplies the grid bias'referred to above, more positive than the negative end of resistance I1, but more negative than the cathode end (24) of this resistance. Meter IB may be used to check the condition of zero grid current. Meter I9 may be used to checkthe self-regulation of the plate current, and the presence of oscillations. y

The triode I I Amay be of any common type, such as type TTY-227 having an amplifying factor of 8. For even better results, a lower factor, such as 3, may be employed. The grid should preferably be uniformly spaced, and suliiciently. extensive to completely control the flow of electrons from cathode to plate. These conditions are met by a grid with relatively 'open uniform spacing, made up of very ne wires. A uni-potential cathode is also preferable such as the indirectly heated type.

Resistance I1 should be from two to twenty times the effective value of the mutual resistance of the tube, under oscillating conditions, it being understood that the mutual resistance is the reciprocal of mutual conductance or, in .other words, is the ratio of change in grid voltage to change in plate current.

The grid-bias voltage 2i may assume any value A from zero to about half the total voltage (20 plus A higher voltage 2| generally requires a higher resistance I1, and likewise causes better self-regulation. v l

The cathode by-pass condensers I5, I6 should be large enough to act as low impedances at the oscillator frequency. They must alsov be small enough to allow the self-regulation to act quickly and prevent periodic blocking of theoscillator. Therefore, the time constant of the total capacity of these two condensers, multiplied by l 2,022,068 a sman'change in plate current, it is desirable, to

the effective mutual resistance of the tube, should be somewhat less than the time constant A of damped free oscillations in the tuned circuit I2, I4.

The feed-back coupling between coils I2- and I3, the resistance I1, and the voltage 2l are i1*- ter-relatcd so that operating conditions are intermediate between the conditions of barely stopping the oscillations and of barely starting grid current. These relations are not critical', but must be satisfied within reasonable limits. In an experimental test of the circuit of Fig. l,

the following circuit values were used: Tube` Il was type UY-22'1; condensers I5 and I6 were each 250 micro-.microfarads; resistor I1 was 100,000 ohms; voltage 20 was 90 volts; voltage 2I was 45 volts. The feed-back coupling was varied over a ratio of 1:4 without stopping oscillations l or starting grid current. 'I'he oscillating grid voltage did not exceed l5 volts. 'I'he plate current (in meter I9) remained between 0.55 and 0.7 milliampere, in spite of this wide variation of feed-back coupling and in spite of the complete absence of overloading effects/such as grid current.

In Fig. 2, a complete oscillating modulator is shown, as used in a supcrhcterodyne receiver. The elements essential to the oscillator will be enumerated first, as follows:

The vacuum tube 4I is a common screen-grid tetrode having plate, screen-grid, controlgrid and cathode. prises coil 44, fixed condenser 43, and .variable tuning condenser 42. 'I'his tuned oscillator cir- 'cuit is coupled to the cathode by mutual inductance between coils 44 and 4G. The tunable oscil- The oscillator tuned circuit comv lation circuit is coupled to the plate or output terminal byv the uniform voltage type coupling comprising the mutual inductance between coils 44 and 45, and the condenser 43 which is common to the tuned circuit and the plate-circuit.

Condenser 41 is an oscillation-frequency by-pass condenser. The resistance 4B is they cathode regulating resistance, common to control-grid, screen-grid and plate circuits. The batteries 35, 36, 31z in series, supply the space current between plate or output electrode and cathode. The tap between batteries 36 and 31. supplies the screen voltage." The tap 38 between batteries 35 and 36 supplies the grid bias voltage. The high resistance grid-leak 33 and small grid condenser 3| prevent excessive grid current, which might other,- wise occur under abnormal operating conditions.

The auxiliary'elements in Fig. 2 which complete the oscillating 'modulator are enumerated as follows:

The input circuit which includes the signalfrequency tuned circuit comprising coil 34 and variable tuning condenser 32, is connected -to the l control-grid and cathode or input terminals of the tube 4I. The incoming signal is coupled to this tuned circuit and is transferred thereby to the control-grid of the oscillating modulator. In

. this manner', a voltage representative of thecurrent to be modulated by the oscillations produced by the tube is impressed between the conftrol grid and cathode thereof. The tunable circuit, which may be preceded by a signal-frequency amplier, if desired, is referred to as the signal input circuit of the oscillating modulator` There are two intermediate-frequency tuned circuits shown. The first comprises c oil 5I and condenser 5 0, connected in the plate circuit of tube 4I. The ysecond is loosely coupled to the iirst, and comprises coi154'and condense-ry 55.

The first circuit 50, I is referred to as the intercalled the second detector of a superheterodyne receiver)l The signal input circuit l32, 34 and the oscillation circuit 42, 43, 44 are tuned simultaneously by a uni-control arrangement of vcondensers 32., 42.

Thesetuned circuits are designed to haveresonant lfrequencies differing by an amount substantially equal to the intermediate frequency. The oscillator nfrequency is preferably the higher. The two tuning condensers 32, 42 generally have thesame capacity variation. The Aoscillator frequencyis made higher by making coil-44 of lower inductance than coil '34. With this change alone, the frequency *difference would 'vary in proportion .tothe resonant frequency of the signal in- 'put circuit. Therefore the difference at higher signal frequencies 'is decreasedby making the eiective minimum capacity of condenser 42 and capacitive coupling, in aiding phase. The inductive coupling is mutual inductance between its associated circuits slightly greater than that of condenser 32 and its associated circuits. Likewise, tlie difference at lower signal frequencies is increased by inserting the fixed condenser '43in series with condenser `42. By proper choice of the oscillator elements 42, 43, 44 relativeto -tle-signal circuit 32,' 34, the frequency difference is made exactly equal t'o the intermediatefrequency I at three 'points' in the tuning range, and vsubstantially constant over the entire tuning range. 'I'his is called the alignment of signal and oscillation circuits Ato secure the intermediate-frequency difference.

The oscillator feed-back is secured by coupling the tuned circuit 42, 43, 44.both to the plate and tothe cathode of the tube 4|. With simple inductive feed-back and condenser tuning, it is well known that with a uniform'ratio of oscillationcircuit voltage to feedback voltage between plate and ground, the loscillation voltage across the tunedcoil 44 decreases at lower frequencies due to the increase `of the equivalent shunt conductance of the tuned circuit. 'In order to make the oscillation-circuit voltage uniform, the ratio of the feedback voltage to the oscillation-circuit voltage is varied by the provisionof a plate coupling 4comprising a compound inductive and coils 44, 45 and the capacitive coupling is condenser 43; The latter has much greater effect atv lower frequencies, because its reactance is greater and the reactance of the other elements 42, 44 is less. `For aiding phase, the unconnected'terminals of coils 44, 45 have opposite alternating polarities relativeto their connected terminals.

The cathode feed-back coupling is mutual inductance between coils 44, 46, which is preferably lless than half the mutual inductance-44, 45. For' oscillation, the cathode and plate have like alternating polarities, the cathode alternating voltage being considerably smaller than that of the plate.

The relations of the oscillator, with regard to are generally the sa'me as deself-regulation, 4The tube 4i may be a screenscribed for Fig. 1.

' grid tube of a common design, such as type UY-I 224. The physical relations preferredfor plate,

grid and cathode in Fig. 1 apply respectively to screen-grid, control-grid andcathodein Fig. 2.

The signal input circuit 32, 34 in Fig. 2, being resonant at a frequency slightly lower than the appreciable inherent directcapacitance between control-grid and cathode in tube 4|, a small part ofthe cathode feed-back voltage is impressed on 5 the control-grid'in a direction to reduce the total feed-back .at higher oscillator frequencies. This effect' isnot serious-if the direct capacitance between control-grid and cathode is kept small,

effect may be as it is in the type UY224 tube. If desired, this 10 eliminated by (a) the use of an extra screen-gridbetween control-grid and cathode, or (b) the neutralization of this direct apacitance .by some means,.such as described in my copendingapplication, Serial No. 739,080, entitledfMethod of and means for eliminating capacitivel coupling, and in my U. S. Patent No. 1,757,494 of the same title.

The coilj5l'acts as a choke coil at the oscillator frequency-. This coil should have a .minimum'20 distributed capacitance. The fundamental .natural frequencyof coilv 5i should be at least half the highest tunable frequency of the oscillator, so that overtones fall outside the oscillator tuning range. This prevents any dissipation which otherwise may occur in choke coils atfrequencies near even multiples of the fundamental fre-Y quency. The condenser 50 shoid be considerably larger than the total other capacitance from plate to ground in tube 4i, coil 5I, and wiring thereto. v'I'his causeslpractically all of the oscillation-frequencyplate current to ilow back to the oscillator-circuit. Also, 'condenser 511 should have, considerably smaller capacit-ance than the total of condensers 4 2, 43. This is to reduce anti-regenerative feed-back from `the intermediate-frequency output circuit to the cathode, vthrough the :the resistance 43 .determines or regulates the negative biasof the grid relative to the cathode and thus regulates plate current and establishes an equilibrium. An increase in the plate current, corresponding to .an increase inthe amplitude of oscillations, tends to increase proportionally the negative bias which opposes the increase in plate current, thus operating to -prevent a change of plate current.. In an oscillator of this type, self-oscillation causes an increase in the average plate current, which, in accordance with the present invention, will cause an increase inthe average grid bias. A new equilibrium point will be reached in which there is slightly greater vaverage plate current and grid bias, and the os cillations are prevented from exceeding this amplitude. 65 By means ofthis arrangement' it is easy to Vestablish a self-regulated oscillation voltage between they grid and cathode which has a peak value,v as the oscillator is tuned throughout its range, which is always smaller than the average '70 grid bias relative to the cathode. In this way the oscillatormo'dulator tube is prevented from' drawing'grid space current, and the circuits are prevented from overloading. The cathode resistance has t e by-pass condenser 41 for oscil- 75 lation-frequency currents, so that the bias is regulated by the average plate current only.

This self-regulating eiect may be made as large as desired, depending upon the sizeof the resistor used. However, in order to have a regulation in which a small change in plate current tends to produce a large change in effective grid bias, the resistor may be proportioned to excessively reduce the space current and a xed positive bias from tap 38 may be applied to the grid relative to the negative or grid end of the cath- \ode resistor to restore the currentsufllciently to negative bias of the cathode resistor and there.

fore the resultant grid bias is always-negative relative to the cathode or common electrode.

The feed-back from the oscillation circuit must be quantitatively related to the cathode resistor. This relation should be such that for a given value of feed-back the resistance is chosen as a mean value between that which would permit the oscillations to swing the grid positive 'and start grid current, andthevalue of resistance which would be required to stop oscillations. With a given resistor the feed-back between the oscillator and grid circuits may be adjusted to a value greater than required to start oscillations but less than required to start grid cur` rent. In other words, the permanently adjusted feed-back coupling means and the resistance are proportioned, relative to each other, to maintain the oscillations automatically at a substantial value less than the minimum value Whichocauses thei grid to swing positive .relative to the cathode and draw current and thus aiiect the tunable input circuit but greater than the minimum value required to sustain oscillations at all frequencies as the oscillation -circuit is tuned throughout itsrange of frequencies; This relationship is not critical, but must be met within reasonable limits It is important thatA the oscillation voltages produced by the oscillation'circuit be substantially uniform vthroughout the entire range to which ihe Aoscillator 'is tuned. For this purpose it is desirable to use an oscillator, as described, in,

which the feed-back is varied automatically byv the use of combined inductive and capacitive couplings.

A high resistance grid-leak 33 anda small' grid condenser 3| are utilized as an additional precuit wasl 812 to 1762 kilocycles. The intermedi'- ate frequency wasl 262 kilocycles, the difference' between signal and oscillator. The tube 4| was microfarads. Coil 34 had 250 microhenrys, and

ycoil 44,160 microhenrys; Mutual inductance 44 I6 was 25 micrhenrys and mutual inductance tu.nng range of the signal input circuit was 550 to 1500 kilocycles, and of the oscillator cir- 45 was 55 microhenrys. Condenser 4 1 was 500 micro-microfarads and resistor 48 was 30,000 ohms. 'Condenser 3| was 50 micro-microfarads and resistor 33 was Yone megohm. Condenser 50 was about 50 micro-microfara-ds and coil El was about 5 millihenrys. 5 The voltages 35, 36, 37 were respectively 30, 6 0

and volts, giving 30 volts on the control-grid,

90 volts on the screen-grid and 180 volts on the plate.

It is to be understood that whereas the oscillator modulator embodying this invention is primarily for the purpose of superheterodyne radio vreceivers in which the grid circuit is coupled to the inputof a radio receiver or the output of a radio-frequency amplifier tube and the plate cirl5 cuit -of the oscillator modulator is coupled to an f intermediate-frequency tuned circuit, the oscillator'modulator described may be utilized inA any other suitable connection, and its use in a superheterodyne is therefore not to be construed as a 20 limitation of the invention.

Furthermore, whereas the above-noted improvements have been found especially useful in radio-frequency circuits, it is to be understood thatthe principles involved are equally applicable 25 for use in connection with vacuum tube circuitsv operating at any desired frequency. Also, the principles involved may be useful in connection with heterodyne, self-heterodyne or autodyne methods of receiving,radio-frequency signals inf30 which the oscillator modulator'produces an audio beat. Similarly, the elements constituting the present invention may be utilized in connection with a homodyne or zero beat receiver. Oscillators of this type may be readily synchronized 35 when tuned approximately to a master oscillator or to a harmonic of a fluisterv oscillator. Os'cillators of this typehave also been found exceptionally free from drifting and from effects of diierent tubes on the oscillation frequency.

What is claimed is:

1. An oscillator-modulator circuit comprising a vacuum tube having a control gridcircuit for receiving a voltage representative of a current to be modulated and an output circuit, a tunable 45 oscillation circuit, means independent of grid current of said tube 'for developing and applying to said grid circuit a bias voltage increasing negatively substantially in proportion to the amplitude of oscillations, and coupling means between said output and grid circuits and said os- -cillation circuit proportioned to feed back to said grid circuit a portion of the oscillation circuit voltage so variable over the tunable range of said oscillation circuit 'as' automatically to 5 maintain said oscillations at a substantial value less than'v the minimum value which causes th grid to swing positive.

2. An oscillator-modulator comprising a vacuum tube' having an anode, a control grid and a cathode and circuits lassociated therewith, said `grid circuit being adapted to receive signal volt'- ages to.be modulated. a tunable oscillation circuit, means for providing an initial negative grid bias increasing negatively in proportion to the amplitude of oscillations, and coupling means between'said anode `and grid circuits and said f oscillation `circuit permanently adjusted to feed'l back to said grid circuit a portion of'the oscil-l lation circuit voltage which is substantially 30' said grid circuit being adapted to receive signal voltages to be modulated, a tunable oscillationV circuit including a tuning device, means for providing an initial negative grid bias increasing negatively in proportion to the amplitude of oscillations, and coupling means between 4said anode Aand grid circuitsand said oscillation circuit permanently adjusted to feed back to said grid circuit a portion of the oscillation circuit voltage y which is substantially greater than that required tosustain oscillations but less than that required to cause'the gridto swing positive relative to.

the cathode over the tunable range of said oscillation circuit, said coupling means including a coupling variable in response to adjustment ofsaid tuning device.

4. An oscillator-modulator comprising a vacu-A 20Ky um tube having an anode, a cathode and a control l `ance cathode bias resistor in said grid circuit tending' excessively to vreduce the space current of said tube, a fixed source of initial positive bias voltage connected -to said grid circuit, said resistor and said source being so proportioned relative to the tube and circuit constants as to maintain the resultant grid bias above that suiilcient to sustain oscillations and below thatwhich causes the grid `to draw grid current..

5. An oscillator-modulator comprising a vacuum tube having an anode, a cathode and a control grid and circuits associated therewith, a-tunable oscillation circuit, said grid circuitbeing adapted to receive signal vvolt-ages to be modulated, coupling means between said anode and grid circuits and said tunable oscillation circuit, a high resistance having one end connected to saidcathode, and a fixed source of grid bias voltage having its positive pole connected to said grid and its negative pole connected to the other end of said resistance, said resistance and voltage being proportioned to sustain the oscillations at a value just below that which causes the grid to swing positive relative to the cathode andthus draw.

grid current.

6. In an oscillator modulator, the combination of a vacuum'tube having input, output and common terminals, an 'input circuit connected to said input and common terminals and including a tunable circuit, a tunable oscillation circuit coupled to said output terminal, said last-mentioned .coupling being of a uniform voltage type, a feedback coupling between: the oscillation circuit and' input circuit, meansfor supplying a bias to the input terminal relative to said common terminal, and a resistor in the lead to the common terminal for regulating said bias, said bias supply means,

said resistor and said last-mentioned feed-back it tosdraw space current and affect said tunable input circuit. i

'7.In an oscillator-modulator, a vacuum tube frequency range.

is produced as the vfrequency of oscillations is 1varied over the tuning range of the oscillation a high resistance having one end connected. to-4 5 said cathode, and a iixed grid'bias meanshaving its Apositive pole connected to said grid and its negative pole connected to thejother end of said resistance, saidresistance, grid bias means-and feed-back coupling being proportioned to sustain the oscillations at a value just below that which causes the grid to swing positive relative to the cathode and thus draw grid current as the oscillation circuit is tuned. l

8. In an oscillator modulator, the combination of a vacuum tube having input, output and common terminals, an' input circuit'connected to said input and common terminals and including a tunable circuit, a tunable oscillation circuit c'oupled to said output terminal, a feed-back coupling 20 between the oscillation circuit and the input clrcuit, means for supplying a bias to theinput terminal relative to said common terminal, a resistor in the lead to the common terminal for regy ulating said bias, said last-mentionedA feed-back 2 coupling being so proportioned that as the oscillation circuit is tuned throughout its range of frequencies the feed-back will be suiiicient at all times to maintain oscillationsbut -insufcient to swing the gridpositive and thus cause itv to draw 3 'grid current and affect the tuned input circuit.

9. In an oscillator modulator, the combination of a vacuum tube having in put,l output and common terminals, an input circuit connected to said input and common terminals and including a 3 tunable circuit,'a,tunable oscillation circuit couvpled to said output terminal, a feed-back coupling between the oscillation circuit and the input circuit, a high resistance vin the leadto the common 'terminal for regulatingthe bias, but which excessively reduces the space current from the common terminal, and means independent of ysaid resistance for providing an input terminal bias vvolt-l -age more positive than thenegative end of said resistance, which bias restores the current sufiiciently to sustain oscillations, said feed-back coupling being so proportioned that as the oscillation circuit is 4tuned throughout its range of frequencies the feed-back will be sufficient at all times to maintain oscillations but insumcie'nt to swing the grid positive and thus cause it 'to draw grid current and affect the tuned input circuit.'

10. In an oscillator modulator, the combination of a vacuum tube having control grid, cathode and plate, means for normally biasing said grid negatively, an input circuit coupled to said control grid and including a tunable circuit, an output circuit connected to said plate, a tuned oscilla- -tion circuit coupled to said output circuit, said coupling including one portion the reactance of`6 which increases with frequency, and a second portion the reactance of which decreases with frequency by a substantially correspondingamount,

' whereby vthe amplitude of the oscillations produced remains substantially constant as the circuit is tuned, and afeed-back coupling between said oscillation circuit and the input circuit, said coupling being so proportioned that the maximum i oscillation voltage impressed upon the input circuit asthe oscillation circuit is tuned shall be insumcient to overcome said' negati-ve bias and swing as said tunable circuits 'are tuned over a desired 75 l1. In an oscillator modulator, the combination of a vacuum tube having control grid, cathode and plate, means for normally biasing said grid negatively, an input circuit coupled to said control'grid and including a tunable circuit, tuned to the frequency of the signals being received, a tuned output circuit connected to said plate, tuned to a fixed'diierence frequency, a tuned uniform-gain oscillation circuit coupled to said output circuit, tuned to a frequency differing from the frequency of the signals being received by said xed differf ence frequency, and a feed-back coupling between said oscillation circuit and the input circuit, said coupling .beingso proportioned that the maximum oscillation voltage impressed upon the input circuit as the oscillation circuit is tuned shall be insufficient to overcome said negative bias and swing the grid positive and thus cause it to draw grid current and yet sufiicient to continue oscillations as said tunable circuits are tuned over a desired frequency range.

12. In an oscillator modulator, the combination of a vacuum tube having control grid, cathode and plate, means for normally biasing said grid quencysaid coupling including a plurality of4 couplings whereby the oscillations produced re-r main substantially constant "as the circuit is tuned, a feed-back coupling between said oscillation circuit and the'input. circuit, said coupling being so proportioned that the maximum oscilla-v tion voltage impressed upon the input circuit as the oscillation circuit is tuned shall be insulcient to overcome said negative bias and swing the grid positive and thus cause it to draw grid current and yet 'suicient to continue oscillations as said tunable circuits are tuned over a desirable frequency range, and means for simultaneously tuning said a tuned oscillation circuit coupled to said output circuit, said coupling including a plurality of couplings, each of said couplings having afreactance which changes oppositely and at a corresponding rate with changes of frequency,` and a feed-back coupling between said oscillation circuit and the input circuit, said coupling being so proportioned that the maximum oscillation voltage impressed upon the input circuit as the oscillation circuit is tuned vshall be insuilicient to overcome said negative bias anclswing the grid positive and thus cause it to draw gridicurre'nt and yet sufiicient to continue oscillations-as said tunable circuits are tuned over 'a desired frequency range.

' HAROLD A. ,WHEELER 

